Modern printed circuit boards are typically laminated from numerous layers, the planar surface of each layer comprising an intricate pattern of conducting regions, formed for example from 1–2 mil thick copper, separated by regions of non-conducting substrate. A fault in any intermediate layer of the board may result in malfunction of the entire board. Consequently, it has become standard practice to check for the existence, integrity and shape of features existing on each of the printed circuit board layers during manufacturing and prior to lamination.
The inspection of complicated printed circuit board layers is generally done optically by machine. The printed circuit board is placed on the machine to enable partial viewing of the board by the collecting optics of an imaging system, and is subsequently scanned. While passing through the field of view of the collecting optics, it is illuminated by an appropriate illumination system.
In prior art systems a single CCD array and illuminator is typically employed. Such conventional arrays and illuminators are typically insufficiently long to acquire an image of the entire width of the printed circuit board in a single pass. As a result, in addition to moving the board and collecting optics relative to each other in a principal scanning direction, the machine must additionally move the board and collecting optics in a second, orthogonal, direction in order to construct an image of the entire board. The result is a composite, image comprised of long thin contiguous strips, on the order of 0.5 mil wide, acquired sequentially from subsequent passes of the CCD array over different sections of the printed circuit board surface. Each strip approximates the field of view of the CCD array.
The acquired image is next analyzed and a map of features on the board is prepared. This resulting map can then be compared, by computer, to a stored map of predetermined features or design rules to which the board is supposed to conform.
Different regions on a printed circuit board may be distinguished by their reflective behavior when exposed to a source of light. For example, the conducting material on a printed circuit board is generally a more specular, if somewhat diffusing reflector of white light relative to the substrate material which is generally more diffuse. Moreover, by relying on differences in spectral reflection properties, it is possible to enhance the contrast between laminate and conductor by using appropriate color filters.
Because image processing of a an image acquired from a printed circuit board relies on an analysis of the reflective properties of its various features, the process can be highly sensitive to the qualities of the light used to illuminate the board. For example, boards are made up of various materials each having differing reflective properties. Additionally, the surface of boards have a topographical relief that may be resultant both from the cross sectional shape of the conductors, as well as surface microstructure. As a result, the intensity or brightness of a reflection of an inspected feature on a board may be dependent not only on the inherent reflective properties of its materials, but also on its surface topography.
To provide an effective illumination in automated optical inspection applications it is necessary to mitigate the effects of topographical variations on a board's surface. Thus, it is known to highly concentrate light along a relatively thin line by using a source configured to emanate light over a relatively wide solid angle of illumination
It is believed that the following patents represent the state of the art in high intensity concentrated illumination for automated inspection of printed circuit boards: U.S. Pat. No. 4,421,410 to Karasaki et al, U.S. Pat. No. 4,877,326 to Chadwick et al, U.S. Pat. No. 4,801,810 to Koso, U.S. Pat. No. 5,058,982 to Katzir et al, and U.S. Pat. No. 5,153,668 to Katzir et al., the disclosures of all of which are incorporated herein by reference.
In some conventional illuminators that provide a wide solid angle concentrated illumination, the strip of the board being inspected is illuminated with light from three linear illumination sources that are fixed substantially parallel to the strip. Light from a first of the illumination sources is concentrated onto the strip from a direction substantially perpendicular to the surface of the board by a cylindrical lens or a section of an elliptical cylindrical mirror running the length of the first light source. Light from a second illumination source is concentrated by a similar lens or mirror onto the strip from a first oblique angle with respect to the normal to the surface. Light from a third illumination source is concentrated similarly onto the strip from a second oblique angle to the normal that is equal and opposite to the first oblique angle. In some of the prior art illuminators, the three illumination sources are configured to create a contiguous solid angle of concentrated light.
For the purposes of clarifying terminology as used herein, it is noted that on-axis illumination is defined as illumination that a reflecting surface parallel to the plane of the workpiece would specularly reflect in a direction along the axis of the collecting optics. Off-axis illumination is defined as illumination that is reflected into the collecting optics by surfaces that are not parallel to the plane of the printed circuit board. In the conventional illuminators, the on-axis illumination illuminates the board from a direction substantially normal to the area of the board being illuminated, while the off-axis illuminators each respectively illuminate the board from directions on either side of the on-axis illumination.
The prior art concentrating broad solid angle illuminators comprise many optical components that must be accurately positioned in order to provide a wide solid contiguous angle of illumination. Settings of the various light sources must also be accurately adjusted. Furthermore these settings and positions must be stabilized and accurately maintained in an environment subject to vibration and large heat transfers. Additionally the “seams” or boundaries between the on-axis illumination and the two off-axis illumination regions are generally defined by the edges of the mirrors or lenses used to concentrate on-axis and off-axis illumination on a board. These seams or boundaries are therefore sharp and generally obtrusive. This makes it difficult to assure that on-axis illumination and off-axis illumination are smoothly blended to provide a substantially uniform illumination throughout the broad angle of illumination over the area of an illuminated strip.
As a result of these difficulties, mechanical and optical components of prior art concentrated illuminators require very tight tolerances and are relatively expensive. Furthermore these difficulties have restricted the lengths of the effective region of illumination to the order of 15 cm, which length is often less than the width of the board being inspected.
An illuminator for providing concentrated light, but having an altogether different design is shown in U.S. Pat. No. 4,801,810. In this patent an elliptical reflector comprising approximately one-half of an elliptical cylinder is used to illuminate the surface of a printed circuit board. The axis of the ellipse is placed at an oblique angle to the surface of the board, with the surface being placed at one focus of the ellipse and a single source of illumination being placed at the second focus. An imaging system images the illuminated line on the board from an angle equal (but opposite) to the angle at which it is directly illuminated by the source. This system provides uneven off-axis illumination of the line on the board and does not allow for independent adjustment of on-axis and off-axis illumination since only a single source is used for illuminating the board from all directions.